1. Field of the Invention
The present invention relates to a power factor correction circuit to correct a power factor when converting an AC input into a DC output.
2. Description of the Related Art
Step-up-type power factor correction circuits have been used to secure a power factor when converting an AC voltage of an AC power source into a DC voltage. For example, Japanese Unexamined Patent Application Publication No. H07-115774 discloses a power source apparatus that arranges high-speed switch elements instead of two rectifying elements on a feedback current passing side of a bridge-type full-wave rectifying circuit and properly controls the high-speed switch elements, to correct a power factor, reduce the number of parts, and increase the conversion efficiency and reliability of the power source apparatus.
The power source apparatus of this related art controls the switch elements based on upper and lower wave segments of a sinusoidal AC line input voltage, to make the waveform and phase of an input current similar to those of the input voltage, thereby correcting a power factor. The related art needs no bridge-type full-wave rectifying circuit, and therefore, causes no loss due to a forward voltage drop of a rectifying diode, thereby improving efficiency.
FIG. 1 is a circuit diagram illustrating a bridge-type power factor correction circuit according to another related art operating in a critical mode. The circuit of FIG. 1 includes reactors L1 and L2, switching elements Q1 and Q2, diodes D1 to D4, capacitors C1 to C3, resistors R1 to R7, half-wave rectifying circuits 1 and 2, and a control circuit 3.
The switching elements Q1 and Q2 are turned on/off at a frequency higher than the frequency of an AC power source AC input, to step-up voltages supplied from main windings L1a and L2a of the reactors L1 and L2.
The control circuit 3 controls the switching of the switching elements Q1 and Q2 in such a way that currents passing through the main windings L1a and L2a of the reactors L1 and L2 have a waveform proportional to the waveform of an AC input voltage of the AC power source AC input, thereby correcting a power factor. The control circuit 3 turns on the switching elements Q1 and Q2 if it is detected according to flyback voltages generated by auxiliary windings L1b and L2b of the reactors L1 and L2 that the currents passing through the main windings L1a and L2a of the reactors L1 and L2 decrease to zero.
The control circuit 3 includes a comparator 4, a one-shot circuit 5, an error amplifier 6, a multiplier 7, a comparator 8, an RS-flip-flop 9, and a driver 10. According to signals to terminals ZCD, CS, FB, and MULT, the control circuit 3 outputs an ON/OFF control signal from a terminal OUT to the switching elements Q1 and Q2.
When the switching elements Q1 and Q2 are turned on, the reactors L1 and L2 accumulate energy, and when the switching elements Q1 and Q2 are turned off, the energy accumulated in the reactors L1 and L2 is stored through the diodes D1 and D2 in the capacitor C2. A voltage across the capacitor C2, i.e., an output voltage is divided by the resistors R3 and R4 and is supplied to the terminal FB of the control circuit 3. In the control circuit 3, the error amplifier 6 compares the voltage at the output voltage detection terminal FB with a reference voltage Vth1, and according to a comparison result, outputs an error voltage to the multiplier 7.
The multiplier 7 multiplies the error voltage from the error amplifier 6 by a voltage at the AC input voltage detecting terminal MULT and outputs a multiplication resultant voltage to the comparator 8. The voltage at the terminal MULT is a voltage formed by rectifying voltages of the main windings L1a and L2a of the reactors L1 and L2 into a pulsating voltage and by dividing the pulsating voltage with the resistors R5 and R6.
The half-wave rectifying circuit 2 half-wave-rectifies voltages generated by the resistors R1 and R2 from currents passing through the switching elements Q1 and Q2 and outputs a voltage to the terminal CS connected to the comparator 8.
The comparator 8 compares the voltage from the half-wave rectifying circuit 2 with the voltage from the multiplier 7, and if the voltage generated by the resistors R1 and R2 is higher than the voltage from the multiplier 7, outputs a signal to a reset terminal R of the RS-flip-flop 9.
The half-wave rectifying circuit 1 half-wave-rectifies flyback voltages generated by the auxiliary windings L1b and L2b from currents passing through the main windings L1a and L2a of the reactors L1 and L2 and provides a rectified voltage through the resistor 7 and terminal ZCD to the comparator 4. The half-wave rectifying circuit 1 removes negative sides of waveforms of different polarities appearing on the auxiliary windings L1b and L2b. Namely, the half-wave rectifying circuit 1 selects only positive-side waveforms in synchronization with the frequency of the AC power source.
The comparator 4 compares the voltage at the terminal ZCD for detecting currents passing through the main windings L1a and L2a of the reactors L1 and L2 with a reference voltage Vth2 and outputs a comparison result signal to the one-shot circuit 5.
If the comparison result signal rises from low to high, the one-shot circuit 5 outputs a low-level signal to a set terminal S of the RS-flip-flop 9. If the comparison result signal falls from high to low, the one-shot circuit 5 outputs a pulse signal having a regular time width to the set terminal S of the RS-flop-flop 9.
Based on the voltage received at the reset terminal R or the set terminal S, the RS-flip-flop 9 carries out a reset operation or a set operation at an output terminal Q thereof. The driver 10 is, for example, a switching circuit using transistors, and based on the voltage at the output terminal Q of the RS-flip-flop 9, turns on/off the switching elements Q1 and Q2.
The control circuit 3 having the above-mentioned configuration achieves a critical mode and detects when flyback voltages generated by the auxiliary windings L1b and L2b of the reactors L1 and L2 become zero, to turn on the switching elements Q1 and Q2. When energy accumulated in the reactors L1 and L2 is discharged to nearly zero, the control circuit 3 starts to accumulate energy in the reactors L1 and L2, thereby maintaining a high utilization rate of the reactors L1 and L2 and making an input current of the AC power source have a sinusoidal waveform that follows the waveform of an input voltage of the AC power source, to correct a power factor.